1) Field of the Invention
The present invention relates generally to an automatic speed changing system for a vehicle employing a two-shaft type gas turbine engine. More specifically, the invention relates to an automatic speed changing system of a two-shaft type gas turbine engine, in which an automatic speed change mechanism capable of varying a reduction ratio in accordance with an engine driving condition is added to an automatic power transmission disposed in a power train between the output shaft of the engine and wheels.
2) Description of the Related Art
Due to the low vibration, wide variation of fuels that can be used, the large torque obtained in a low engine speed range, and other advantages, studies of the practical use of two-shaft type gas turbine engines have been made as an important factor in automotive technologies. FIG. 1 shows the construction of a typical two-shaft type gas turbine engine employed in an automotive vehicle having an automatic power transmission.
In such a two-shaft type gas turbine engine, the engine is started by a starter motor SM with a built-in clutch for driving a front gear F/G, and intake air is compressed by a compressor C, heated by a heat exchanger HE, and combusted in a combustion chamber CC by mixing with a fuel supplied from an actuator Al. The combustion gas thus-generated drives a compressor turbine CT arranged coaxially with the compressor C. The compressor turbine CT and the compressor C as combined will be referred to hereafter as "gas generator GG". The compression at the compressor C is varied depending upon the revolution speed of the compressor turbine CT. The thus-used combustion gas for driving the compressor turbine CT is transferred through a variable nozzle VN having a combustion gas path area which is adjusted by an actuator A2, for driving a power turbine PT. Then, the combustion gas passes through the heat exchanger HE and is exhausted to the atmosphere as an exhaust gas.
The actuators A1 and A2 are controlled by a control circuit CONT in accordance with a driving condition of the engine, and accordingly, a depression magnitude of an accelerator pedal and engine driving parameters are input from sensors (not shown) to the control circuit CONT. In FIG. 1, encircled figures indicate the intake air pressure P and temperature T, and represent the intake air and temperature at the corresponding position.
In the two-shaft type gas turbine engine having the construction set forth above, the revolution speed N2 of the power turbine PT is reduced by the reduction gear to a rotation speed N3, and the reduced rotation speed N3 is then transferred to an automatic power transmission A/T coupled with a torque converter including a lock-up clutch L/C. The automatic power transmission A/T converts the rotation speed to a transmission speed ratio corresponding to the shift position thereof, and transfers the rotation to the wheel W via a differential gear unit D.
The vehicle employing the two-shaft type gas turbine engine encounters a problem of obtaining a good acceleration immediately after starting from a standstill. This problem will be discussed with reference to FIG. 2.
FIG. 2 shows a comparison of an accelerating ability from standstill of a vehicle employing the gas turbine engine and that of a vehicle employing a reciprocating gasoline engine. It should be noted that the rated output of the gasoline engine is the same as that of the two-shaft type gas turbine engine. As can be seen from the figure, during the initial 0 to 2 seconds from of the start of an acceleration from a standstill of the vehicle, the two-shaft type gas turbine engine vehicle has lower response characteristics than that of the reciprocating gasoline engine vehicle, but after 2 seconds, the acceleration characteristic of the two-shaft type gas turbine engine vehicle becomes equivalent to that of the reciprocating gasoline engine vehicle. Therefore, it can be appreciated that the two-shaft type gas turbine engine vehicle has a particularly low acceleration response characteristic at the initial stage of an acceleration.
The reason for the low response characteristic will be discussed herebelow. FIGS. 3A to 3C illustrate acceleration characteristics from a standstill of the two-shaft type gas turbine engine vehicle. At a time t&lt;0, the two-shaft type gas turbine engine is in an idling condition and the vehicle is at a standstill. It is assumed that the accelerator pedal is fully depressed at a timing t&gt;0, and thereafter, from a timing t=0 the fuel flow rate Gf is controlled so that the inlet temperature of the gas generator GG becomes a target value, e.g., 1100.degree. C. At the same time the variable nozzle VN is controlled to obtain an optimum acceleration both at the gas generator GG and of the vehicle. During a period 0&lt;t&lt;t1, in which the engine is accelerated to the rated rotation speed, the gas generator GG must be accelerated, and accordingly, the compressor turbine CT must be driven at a higher speed than the rotation speed in a normal driving condition, at a magnitude sufficient to accelerate the gas generator GG, and therefore, it becomes necessary to correspondingly lower the output power of the power turbine PT. When the path area of the variable nozzle is reduced to obtain a better vehicular acceleration while the gas generator GG is in the accelerating state, the output of the compressor turbine CT is lowered, and thus the acceleration at the gas generator GG is to further delay the timing t1, and therefore, the vehicular acceleration characteristics are worsened. Namely, there is an optimal range in the control of the variable nozzle which at the same time achieves a vehicular acceleration of a certain magnitude. The resultant acceleration characteristics derived from the optimal control for the variable nozzle VN are illustrated in FIG. 2. Therefore, it has been an established theory that the two-shaft type gas turbine engine vehicle has low acceleration response characteristics at the initial stage of an acceleration from a standstill.